Methods and apparatus exist in the art for interconnecting messaging systems such as, for example, a voice mail system, with a private branch exchange ("PBX"). For example, U.S. Pat. No. 4,873,718 discloses a method and apparatus for interconnecting a voice messaging system with a PBX which entails use of a multiple line feature telephone interface for retrieving call control information. The interconnection that is made in accordance with the disclosure is illustrated in FIG. 1. This disclosure has a disadvantage in that the interconnection requires an actual feature telephone and, in accordance with the disclosure, the voice messaging system utilizes a single line connection for voice transfer and the voice messaging system taps into the display of the feature telephone to retrieve control information which is routed to the voice messaging system. In addition, the use of a single line connection has a further disadvantage in that there is lack of redundancy and there is decreased bandwidth since control information transmitted from the PBX is limited by the hardware and associated bandwidth of a single feature phone. A still further disadvantage is that analog transmissions are required. This complicates line handling since analog balancing is required.
Further, U.S. Pat. No. 4,926,462 to Ladd, et. al. discloses a method and apparatus for interconnecting a voice messaging system 250 with a PBX 210 wherein the interface is divided into two (2) parts; a feature phone interface 212 and a station interface unit 213. The interconnection that is made in accordance with the disclosure is illustrated in FIG. 2. In the disclosed interconnection configuration, the voice messaging system 250 appears to the PBX both as an actual feature phone having a multiple line capacity at extension numbers 241 and as a set of analog, single line telephones at voice port lines 240. The particular type of data link utilized to couple the PBX with the feature phone interface necessarily varies with the type of manufacturer. In accordance with the disclosure, if extension A221 calls extension B222 and is forwarded due to, for example, a ring-no-answer condition to a voice messaging system, the voice messaging system will receive call information from the PBX through the feature phone interface 212. The voice messaging system will go off-hook on an appropriate voice port 252 and process the call. Alternatively, a voice port may be conferenced into the call and the feature phone interface will subsequently be disconnected. While the voice port processes the call, calls arriving on other extensions via station interface unit 214 or calls received from external phones 201 via a central office 203 and truck interface unit 208 may be answered by the feature phone emulator interface 251 and processed in a similar manner. The external calls are received over truck groups 205 and 206. This disclosure has a disadvantage in that control information for a given call is transmitted over a separate link 241 which must be dedicated to handling control information. Further, there is a lack of redundancy and decreased bandwidth since control information transmitted from the PBX is limited by the hardware and associated bandwidth of a single feature phone. A second disadvantage is that analog transmissions are required. This complicates line handling since analog balancing is required.
In light of the above, there is a need in the art for a method and apparatus for interconnecting a messaging system (voice, facsimile, and so forth) with a PBX which provides entirely digital voice transmission and which provides high bandwidth and redundant transmission of control information between the messaging system and the PBX.